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  <div class="section" id="built-in-constants">
<span id="built-in-consts"></span><h1>3. Built-in Constants<a class="headerlink" href="#built-in-constants" title="Permalink to this headline">¶</a></h1>
<p>A small number of constants live in the built-in namespace.  They are:</p>
<dl class="data">
<dt id="False">
<tt class="descname">False</tt><a class="headerlink" href="#False" title="Permalink to this definition">¶</a></dt>
<dd><p>The false value of the <a class="reference internal" href="functions.html#bool" title="bool"><tt class="xref py py-class docutils literal"><span class="pre">bool</span></tt></a> type. Assignments to <tt class="docutils literal"><span class="pre">False</span></tt>
are illegal and raise a <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a>.</p>
</dd></dl>

<dl class="data">
<dt id="True">
<tt class="descname">True</tt><a class="headerlink" href="#True" title="Permalink to this definition">¶</a></dt>
<dd><p>The true value of the <a class="reference internal" href="functions.html#bool" title="bool"><tt class="xref py py-class docutils literal"><span class="pre">bool</span></tt></a> type. Assignments to <tt class="docutils literal"><span class="pre">True</span></tt>
are illegal and raise a <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a>.</p>
</dd></dl>

<dl class="data">
<dt id="None">
<tt class="descname">None</tt><a class="headerlink" href="#None" title="Permalink to this definition">¶</a></dt>
<dd><p>The sole value of the type <tt class="docutils literal"><span class="pre">NoneType</span></tt>.  <tt class="docutils literal"><span class="pre">None</span></tt> is frequently used to
represent the absence of a value, as when default arguments are not passed to a
function. Assignments to <tt class="docutils literal"><span class="pre">None</span></tt> are illegal and raise a <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a>.</p>
</dd></dl>

<dl class="data">
<dt id="NotImplemented">
<tt class="descname">NotImplemented</tt><a class="headerlink" href="#NotImplemented" title="Permalink to this definition">¶</a></dt>
<dd><p>Special value which should be returned by the binary special methods
(e.g. <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><tt class="xref py py-meth docutils literal"><span class="pre">__eq__()</span></tt></a>, <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><tt class="xref py py-meth docutils literal"><span class="pre">__lt__()</span></tt></a>, <a class="reference internal" href="../reference/datamodel.html#object.__add__" title="object.__add__"><tt class="xref py py-meth docutils literal"><span class="pre">__add__()</span></tt></a>, <a class="reference internal" href="../reference/datamodel.html#object.__rsub__" title="object.__rsub__"><tt class="xref py py-meth docutils literal"><span class="pre">__rsub__()</span></tt></a>,
etc.) to indicate that the operation is not implemented with respect to
the other type; may be returned by the in-place binary special methods
(e.g. <a class="reference internal" href="../reference/datamodel.html#object.__imul__" title="object.__imul__"><tt class="xref py py-meth docutils literal"><span class="pre">__imul__()</span></tt></a>, <a class="reference internal" href="../reference/datamodel.html#object.__iand__" title="object.__iand__"><tt class="xref py py-meth docutils literal"><span class="pre">__iand__()</span></tt></a>, etc.) for the same purpose.
Its truth value is true.</p>
</dd></dl>

<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>When <tt class="docutils literal"><span class="pre">NotImplemented</span></tt> is returned, the interpreter will then try the
reflected operation on the other type, or some other fallback, depending
on the operator.  If all attempted operations return <tt class="docutils literal"><span class="pre">NotImplemented</span></tt>, the
interpreter will raise an appropriate exception.</p>
<p class="last">See
<a class="reference internal" href="numbers.html#implementing-the-arithmetic-operations"><em>Implementing the arithmetic operations</em></a>
for more details.</p>
</div>
<dl class="data">
<dt id="Ellipsis">
<tt class="descname">Ellipsis</tt><a class="headerlink" href="#Ellipsis" title="Permalink to this definition">¶</a></dt>
<dd><p>The same as <tt class="docutils literal"><span class="pre">...</span></tt>.  Special value used mostly in conjunction with extended
slicing syntax for user-defined container data types.</p>
</dd></dl>

<dl class="data">
<dt id="__debug__">
<tt class="descname">__debug__</tt><a class="headerlink" href="#__debug__" title="Permalink to this definition">¶</a></dt>
<dd><p>This constant is true if Python was not started with an <a class="reference internal" href="../using/cmdline.html#cmdoption-O"><em class="xref std std-option">-O</em></a> option.
See also the <a class="reference internal" href="../reference/simple_stmts.html#assert"><tt class="xref std std-keyword docutils literal"><span class="pre">assert</span></tt></a> statement.</p>
</dd></dl>

<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The names <a class="reference internal" href="#None" title="None"><tt class="xref py py-data docutils literal"><span class="pre">None</span></tt></a>, <a class="reference internal" href="#False" title="False"><tt class="xref py py-data docutils literal"><span class="pre">False</span></tt></a>, <a class="reference internal" href="#True" title="True"><tt class="xref py py-data docutils literal"><span class="pre">True</span></tt></a> and <a class="reference internal" href="#__debug__" title="__debug__"><tt class="xref py py-data docutils literal"><span class="pre">__debug__</span></tt></a>
cannot be reassigned (assignments to them, even as an attribute name, raise
<a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a>), so they can be considered &#8220;true&#8221; constants.</p>
</div>
<div class="section" id="constants-added-by-the-site-module">
<h2>3.1. Constants added by the <a class="reference internal" href="site.html#module-site" title="site: Module responsible for site-specific configuration."><tt class="xref py py-mod docutils literal"><span class="pre">site</span></tt></a> module<a class="headerlink" href="#constants-added-by-the-site-module" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="site.html#module-site" title="site: Module responsible for site-specific configuration."><tt class="xref py py-mod docutils literal"><span class="pre">site</span></tt></a> module (which is imported automatically during startup, except
if the <a class="reference internal" href="../using/cmdline.html#cmdoption-S"><em class="xref std std-option">-S</em></a> command-line option is given) adds several constants to the
built-in namespace.  They are useful for the interactive interpreter shell and
should not be used in programs.</p>
<dl class="data">
<dt id="quit">
<tt class="descname">quit</tt><big>(</big><em>code=None</em><big>)</big><a class="headerlink" href="#quit" title="Permalink to this definition">¶</a></dt>
<dt id="exit">
<tt class="descname">exit</tt><big>(</big><em>code=None</em><big>)</big><a class="headerlink" href="#exit" title="Permalink to this definition">¶</a></dt>
<dd><p>Objects that when printed, print a message like &#8220;Use quit() or Ctrl-D
(i.e. EOF) to exit&#8221;, and when called, raise <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><tt class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></tt></a> with the
specified exit code.</p>
</dd></dl>

<dl class="data">
<dt id="copyright">
<tt class="descname">copyright</tt><a class="headerlink" href="#copyright" title="Permalink to this definition">¶</a></dt>
<dt id="license">
<tt class="descname">license</tt><a class="headerlink" href="#license" title="Permalink to this definition">¶</a></dt>
<dt id="credits">
<tt class="descname">credits</tt><a class="headerlink" href="#credits" title="Permalink to this definition">¶</a></dt>
<dd><p>Objects that when printed, print a message like &#8220;Type license() to see the
full license text&#8221;, and when called, display the corresponding text in a
pager-like fashion (one screen at a time).</p>
</dd></dl>

</div>
</div>


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  <h3><a href="../contents.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">3. Built-in Constants</a><ul>
<li><a class="reference internal" href="#constants-added-by-the-site-module">3.1. Constants added by the <tt class="docutils literal"><span class="pre">site</span></tt> module</a></li>
</ul>
</li>
</ul>

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